The present invention relates generally to data communication systems and more particularly relates to an apparatus for and method of synchronizing in a spread spectrum communication system.
The use of spread spectrum communications techniques to improve the reliability and security of communications is well known and is becoming more and more common. Spread spectrum communications transmits data utilizing a spectrum bandwidth that is much greater than the bandwidth of the data to be transmitted. This provides for a more reliable communication in the presence of high narrowband noise, spectral distortion and pulse noise, in addition to other advantages. Spread spectrum communication systems typically utilize correlation techniques to identify an incoming received signal.
Spread spectrum communications systems are commonly used in military environments to overcome high-energy narrowband enemy jamming. In commercial or home environments it may be used to achieve reliable communication on noise media such as the AC powerline. In particular, certain home electrical appliances and devices can potentially be very disruptive of communications signals placed onto the powerline. For example, electronic dimming devices can place large amounts of noise onto the powerline since these devices typically employ triacs or silicon controlled rectifiers (SCRs) to control the AC waveform in implementing the dimming function.
A communication medium such as the AC powerline may be corrupted by fast fading, unpredictable amplitude and phase distortion and additive noise. In addition, communication channels may be subjected to unpredictable time varying jamming and narrowband interference. In order to transmit digital data over such channels it is preferable to use as wide a bandwidth as possible for transmission of the data. This can be achieved using spread spectrum techniques.
One common type of spread spectrum communications, called direct sequence spread spectrum, is generated by first modulating the digital data and then multiplying the result with a signal having a particularly desirable spectral properties, such as a Pseudo Noise (PN) sequence. The PN sequence is a periodic sequence of bits having a particular period. Each bit in the sequence is termed a chip. The sequence has the property of having very low autocorrelation for delays larger than one chip. In some systems, the PN sequence is replaced by a chirp waveform. Several techniques are available for the transmitter to modulate the data signal, including biphase shift keying also referred to as bipolar phase shift keying (BPSK) and continuous phase modulatation (CPM) techniques. Minimum shift keying (MSK) is a known variation of CPM.
The spread spectrum receiver is required to perform synchronization that is commonly implemented using an acquisition method in combination with a tracking loop or other tracking mechanism. In a noisy unpredictable environment such as the AC powerline, the tracking loop typically fails frequently causing loss of information. Communication systems to overcome these problems are large, complex and expensive. In addition, these systems typically succeed at transmitting only one or more bits per symbol.
Receiving and demodulating signals that have been subject to PN modulation requires that the same PN code sequence be generated in the receiver and correlated with the received signal to extract the data modulation. One type of correlation technique employs a digital matched filter to compare the received digital signal with the locally generated version of the PN code. The digital filter produces an in phase (I) signal and a quadrature (Q) signal from which a digital demodulator such as a DPSK demodulator can derive data values. Another function of the digital matched filter is to produce correlation measurements from which synchronization signals can be generated.
In despreading a spread spectrum signal, the receiver produces a correlation pulse in response to the received spread spectrum signal when the received spread spectrum signal matches the chip sequence to a predetermined degree. Various techniques are available for correlating the received signal with the chip sequence, including those using surface acoustic wave (SAW) correlators, tapped delay line (TDL) correlators, serial correlators, and others.
Synchronization of signals between a transmitter and receiver that are communicating with each other in a spread spectrum communication system is an important aspect of the process of transmitting signals between them. Synchronization between transmitter and receiver is necessary to allow the despreading of the received signals by a spreading code that is synchronized between them so that the originally transmitted signal can be recovered from the received signal. Synchronization is achieved when the received signal is accurately timed in both its spreading code pattern position and its rate of chip generation with respect to the receiver""s spreading code.
One of the problems associated with synchronization is that the techniques used to synchronize two signals are relatively expensive to implement. In communication systems having sophisticated and relatively expensive central communication sites which serve a plurality of relatively inexpensive remote communication sites, it is desirable to reduce the cost of synchronization systems in the remote communication sites while not increasing the cost of the central communication sites.
In spread spectrum systems two general areas of uncertainty of the signal exist which must be resolved before a received spread spectrum signal can be recovered. These areas of uncertainty are spreading code phase and carrier frequency. In addition, spreading code clock rate can be a source of synchronization uncertainty. Most of this uncertainty may be eliminated by utilizing accurate frequency sources in both transmitter and receiver that are communicating with each other. However, some uncertainty cannot be eliminated by the use of accurate frequency sources, e.g., Doppler-related frequency errors.
One well-known synchronization technique involves using a sliding correlator. In the sliding correlator, a spreading code generator operates at a rate different from the rate at which a spreading code generator associated with a transmitter that transmitted the signal to be correlated operates. The effect is that the two spreading code sequences slip in phase with respect to each other, and if viewed simultaneously, the spreading codes would seem to slide past each other until the point of coincidence is reached.
More particularly, a sliding correlator receives a spread spectrum signal that is a function of a particular spreading code and generates a signal locally, which is a function of a locally-generated spreading code that is substantially similar to the particular spreading code. Subsequently, the sliding correlator compares the received signal with the locally generated signal. If the two signals are not determined to be aligned, then the sliding correlator phase shifts the local signal with respect to the received signal and loops back to compare the phase shifted local signal with the received signal. This process continues until the sliding correlator determines that the two signals are aligned at which point the total phase shift of the local signal is stored by the sliding correlator for subsequent use. The total phase shift and the locally-generated spreading code are used to despread subsequently received spread spectrum signals which have been spread with spreading codes which are substantially similar to the locally generated spreading code, but phase-shifted.
In general, prior art receivers that utilize standard synchronization techniques such as matched filter detectors or sliding matched filter detectors may not be able to receive if the channel distorts the transmitted waveform sufficiently such that the waveform at the receiver side does not match or badly matches the original sequence.
The present invention is an apparatus and method for acquiring synchronization, i.e., acquiring the presence of a packet of data and associated timing information, for use in communications systems regardless of the type of modulation used. Examples of types of applicable communications receivers include BPSK, QAM and even OFDM receivers. A typical application of the invention, however, is in spread spectrum data communications systems that utilize the Differential Code Shift Keying (DCSK) or non-differential Code Shift Keying (CSK) modulation technique. The invention is particularly useful in general in any application where (1) synchronization needs to be performed quickly; (2) the synchronizer has no (or not enough) a priori knowledge of where to expect synchronization thus preventing a tracking loop from being used and (3) there is enough distortion, linear or nonlinear, such that correlating with a known signal does not work or works poorly.
An assumption of the present invention is that the properties of a communications channel do not change that rapidly compared to the time needed to transmit a packet of data. Based on this assumption, the invention functions to transmit a sequence of symbols having known rotation and phase to the receiver. The receiver attempts to match the received vectors in a predefined manner in order to determine whether a signal or noise is being received.
Such communications systems are applicable to relatively noisy and/or distorting environments such as the AC powerline, telephone line (low quality or unmatched twisted pair), wireless (e.g., RF and IR). The present invention utilizes CSK modulation for the synch acquisition stage. Once synchronization is obtained, the modem can switch to any desired data carrying modulation, which may or may not be CSK.
In a CSK transmission system, the data is transmitted in the form of time shifts between consecutive circularly rotated waveforms of length T which are referred to as spreading waveforms. The spreading waveforms can comprise any type of waveform that has suitable auto correlation properties.
During each symbol period, referred to as a unit symbol time (UST), a plurality of bits are transmitted. The symbol period is divided into a plurality of shift indexes with each shift index representing a particular bit pattern. The information, i.e., bit pattern, is conveyed by rotating the spreading waveform by a certain amount corresponding to the data to be transmitted. The data is conveyed in the degree of rotation or circular shift applied to the spreading waveform (also referred to as a chirp) before it is transmitted.
In a CSK system, the data is conveyed in the absolute shift assigned to the spreading waveform. In a DCSK system, the data is conveyed in the shift differential between consecutive symbols. The synchronization scheme of the present invention is applicable to both CSK and DCSK transmission systems.
A correlator in the receiver employs a matched filter having a template of the spreading waveform pattern to detect the amount of rotation (or circular shift) within the received signal for each symbol. The received data is fed into a shift register and circularly rotated, i.e., shifted. For each bit shift or rotation, the matched filter generates a correlation sum. A shift index is determined for each UST corresponding to the shift index that yields the maximum (or minimum) correlation sum. Differential shift indexes are generated by subtracting the currently received shift index from the previously received shift index. The differential shift index is then decoded to yield the originally transmitted data.
Note that it is assumed that the transmitter transmits data in the form of packets to the receiver. In the present invention, each packet is preceded by a preamble comprising a number of symbols. The length of the preamble can be any suitable number of symbols such that the receiver is able to synchronize with the transmitter. The preamble comprises a sequence of rotated or non-rotated symbols, inverted or non-inverted (or generally phase-rotated by some amount). In some embodiments, the preamble comprises a sequence of non-rotated symbols (or symbols with a constant fixed rotation) followed by one or more symbols with a known random shift, the rotation applied to each rotated symbol being independent of the rotation applied to other symbols. Embodiments are described herein that utilize both types of preambles.
The present invention is first described followed by the presentation of several embodiments that illustrate the principles of the present invention. In a first embodiment, the template comprises a number of shift registers equal to the number of non-rotated symbols in the preamble. In the second and third embodiments, the template comprises a number of shift registers that is independent of the number of symbols transmitted in the preamble. Note that the preamble used in the first embodiment may have two or more rotated symbols.
In the first, second and third embodiments, the rotated symbol is detected by clocking the received signal into a shift register at a tap position located a number of taps from the first tap. The tap position corresponds with the amount of shift applied to the symbol at the transmitter. After the symbol is received and clocked for a UST period, the shift register holds a non-rotated symbol that is correlated with the contents of the template. Under normal operation, the template contains a function of the plurality of non-rotated symbols. Thus, upon receipt of the shifted symbol, the correlator generates a peak sum that is detected and synchronization is subsequently declared.
In a second embodiment, the number of template shift registers is independent of the number of symbols in the preamble and a single shifted symbol is transmitted. A third embodiment is similar to the second with the difference being two shifted symbol are transmitted. The synchronization apparatus thus comprises additional circuitry to correlator two symbols simultaneously.
In a fourth embodiment, the number of shift registers is equal to the number of symbols in the preamble. A plurality of functions processes the portions of the shift registers. The outputs of these functions are input to an accumulator. When a proper preamble sequence is received, the accumulator generates a peak and in response, a synchronization pulse is output.
There is provided in accordance with the present invention, in a communications system, a method of acquiring synchronization, comprising the steps of transmitting a preamble over a communication channel, the preamble comprising a plurality of symbols having known rotation and phase, receiving a sequence of input samples from the communication channel, dividing the received sequence into two or more vectors, de-rotating and phase correcting the vectors back to their original rotation and phase and applying a matching function between the vectors so as to generate a metric indicative of the degree of synchronization.
The symbols may comprise spreading waveforms characterized by high, sharp autocorrelation or pseudo noise sequences. The step of applying a matching function may utilize a correlator to compare the one or more vectors and a template formed from some of the vectors in a recursive fashion or a non-recursive fashion.
There is also provided in accordance with the present invention, in a communications system, a method of acquiring synchronization, comprising the steps of transmitting a preamble over a communication channel, the preamble comprising a plurality of symbols having known rotation and phase, receiving a sequence of input samples from the communication channel over a plurality of channels, each channel corresponding to a different frequency band, dividing the received sequence into two or more vectors within each channel, de-rotating and phase correcting the vectors back to their original rotation and phase, applying a matching function between the vectors so as to generate a metric for each channel indicative of the degree of synchronization and combining the metric generated for each the channel so as to produce a combined output metric indicative of the degree of synchronization for all channels.
The step of combining may comprise generating the combined output metric utilizing separate synchronization circuits, each synchronization circuit associated with a single channel, wherein the output of one of the synchronization circuits is selected according to a predetermined criterion. The step of combining may also comprise the steps of passing the output of each channel through a nonlinear function and summing the outputs of the nonlinear function for each channel.
There is further provided in accordance with the present invention, in a communications system, a method of acquiring synchronization, comprising the steps of transmitting a preamble comprising a plurality of symbols having a first rotation and phase followed by one or more rotated symbols each having a predefined rotation and phase, the preamble transmitted over a communications channel, generating a template adapted in accordance with the contents of the plurality of symbols during the reception thereof, generating vectors from a signal received from the communications channel, the vectors generated in accordance with the rotated symbols, de-rotating and phase correcting the vectors back to their original rotation and phase and matching the vectors with the template so as to generate a metric indicative of the degree of synchronization.
The step of generating vectors comprises the step of inputting the received signal into a shift register at a tap position a distance from the first tap corresponding to the amount of a second rotation. The step of matching the vectors comprises the step of correlating the contents of the template with the contents of a shift register so as to generate a correlation sum, and detecting synchronization in response to a maximum correlation sum.
There is still further provided in accordance with the present invention an apparatus for acquiring synchronization in a communications system, the communications system including a preamble consisting of a plurality of symbols having a first rotation followed by a shifted symbol having a second rotation, the preamble transmitted over a communications channel, the apparatus comprising a template including means for adapting the template to the characteristics of the communication channel during the reception of the plurality of symbols, a shift register adapted to input a received signal at a tap position a distance from the first tap corresponding to the amount of the second rotation and a correlator operative to correlate the contents of the template with the contents of the shift register so as to generate a correlation sum, synchronization being detected in response to a maximum correlation sum.
There is also provided in accordance with the present invention, in a communications system, a method of acquiring synchronization, comprising the steps of transmitting a preamble comprising a plurality of symbols having a first rotation followed by M shifted symbols, wherein each shifted symbol is shifted by an amount independent of the shifts of other symbols, the preamble transmitted over a communications channel, generating a template that is adapted to the characteristics of the communication channel during the reception of the plurality of symbols, inputting a received signal into an Ith shift register at a tap position a distance from the first tap corresponding to the amount of rotation of the Ith shifted symbol, correlating the contents of the template with the contents of the M shift registers so as to generate a plurality of correlations and summing the plurality of correlations to generate a correlation sum and detecting synchronization in response to a maximum correlation sum.
In addition, there is provided in accordance with the present invention an apparatus for acquiring synchronization in a communications system, the communications system including a preamble consisting of a plurality of symbols having a first rotation followed by M shifted symbols, wherein each shifted symbol is shifted by an amount independent of the shifts of other symbols, the preamble transmitted over a communications channel, the apparatus comprising a template including means for adapting the template to the characteristics of the communication channel during the reception of the plurality of symbols, M shift registers, the Ith shift register adapted to input a received signal at a tap position a distance from the first tap corresponding to the amount of rotation of the Ith shifted symbol, M correlators operative to correlate the contents of the template with the contents of the M shift registers so as to generate M correlation outputs and a summer for summing the M correlation outputs so as to generate a correlation sum, synchronization being detected in response to a maximum correlation sum.
There is further provided in accordance with the present invention an apparatus for acquiring synchronization in a communication system, the communication system including a preamble sequence consisting of a plurality of symbols wherein each symbol has either a zero shift or a non-zero shift rotation associated therewith, the apparatus comprising N shift registers wherein N is equal to the number of symbols in the preamble sequence, a first set of M functions wherein each function comprises N inputs, one input from each shift register, the first set of M functions for processing samples input to the left most tap of the N shift registers and for processing samples input to the tap position corresponding to those symbols having non-zero shift rotations, a second set of M functions wherein each function comprises N inputs, one input from each shift register, the second set of M functions for processing samples output from the right most tap of the N shift registers and for processing samples output from tap positions one before the tap positions corresponding to those symbols having non-zero shift rotations, an accumulator coupled to the output of each the function in the first set of M functions and the second set of M functions, wherein the output of the first set of M functions is added to the accumulator and the output of the second set of M functions is subtracted from the accumulator and maximum detector circuitry operative to declare synchronization in response to a maximum peak output of the accumulator during a symbol time period.
The function comprises summing all N inputs and squaring the result. The apparatus further comprises means for phase correcting the symbols in accordance with a phase rotation or phase inversion dj previously applied to the symbols in the preamble.
There is also provided in accordance with the present invention, in a communication system including a preamble sequence consisting of a plurality of symbols wherein each symbol has either a zero shift or a non-zero shift rotation associated therewith, a method of acquiring synchronization, the method comprising the steps of transmitting a preamble comprising a plurality of symbols, each symbol having either a zero rotation shift or a non-zero rotation shift, wherein each non-zero shifted symbol is shifted by an amount independent of the shifts of other symbols, the preamble transmitted over a communications channel, inputting a received signal into a set of N shift registers where N is equal to the number of symbols in the preamble sequence, processing selected taps of the N shift registers via a first set of M functions wherein each function comprises N inputs, one input from each shift register, the first set of M functions for processing samples input to the left most tap of the N shift registers and for processing samples input to the tap position corresponding to those symbols having non-zero shift rotations, processing selected taps of the N shift registers via a second set of M functions wherein each function comprises N inputs, one input from each shift register, the second set of M functions for processing samples output from the right most tap of the N shift registers and for processing samples output from the tap positions one before the tap positions corresponding to those symbols having non-zero shift rotations, accumulating an accumulator value by adding the output of the first set of M functions to the accumulator value and subtracting the output of the second set of M functions from the accumulator value and declaring synchronization in response to the occurrence of a maximum peak of the accumulator value during a symbol time period.
There is further provided in accordance with the present invention, in a communications system, a method of acquiring synchronization, comprising the steps of transmitting a preamble over a communication channel, the preamble comprising a plurality of symbols having known time shift and phase, receiving a sequence of input samples from the communication channel, dividing the received sequence into two or more vectors, correcting the vectors in time and phase back to their original time shift and phase and applying a matching function between the vectors so as to generate a metric indicative of the degree of synchronization.
There is also provided in accordance with the present invention, in a communications system, a method of acquiring synchronization, comprising the steps of transmitting a preamble over a communication channel, the preamble comprising a plurality of symbols having known rotation, time shift and phase, receiving a sequence of input samples from the communication channel, dividing the received sequence into two or more vectors, de-rotating, de-shifting and phase correcting the vectors back to their original rotation, time shift and phase and applying a matching function between the vectors so as to generate a metric indicative of the degree of synchronization.